1. Field of the Invention
The present invention relates to a catadioptric objective system, and particularly to a catadioptric objective system which is used for forming a micro beam spot, and more particularly to a catadioptric objective system which is suited to be used for storage and/or reproduction of information by use of an optical near field.
2. Description of Related Art
In the art of optical storage/reproduction, in order to form a micro beam spot, generally, it is desired to converge a light beam to the diffraction limit by use of an optical system with the highest possible numerical aperture. However, the minimization of a beam spot by use of conventional lenses has reached the limit. This means that objective lenses which propagate light in the air have been improved to have numerical apertures of almost 1.0.
It is possible to heighten the limit of a beam spot diameter by providing a dielectric around the converging point, and Japanese Patent Laid-Open Publication Nos. 2002-48973 (reference 1) and 2000-162503 (reference 2) suggest the effective numerical aperture be heightened by use of a solid immersion lens made of a high-refractive material.
Both of the solid immersion lenses disclosed by the references 1 and 2 converge a light on their respective lens surfaces. When storage and/or reproduction of information are carried out by use of an optical near field, the distance between the lens surface (converging point) and a storage medium must be controlled within a very small value less than a wavelength of light. However, since the lens surface has a curvature, it is difficult to measure and control the very small distance between the converging point and the storage medium.
The reference 1 suggests a small opening be formed around the converging point on the lens surface. However, a process of forming a fine structure such as a very small opening on the curved lens surface by use of an electronic beam includes a very difficult work such as focusing of the electronic beam.
The solid immersion mirror disclosed by the reference 1 is composed of a plane surface (incident surface) and a convex surface, that is, the solid immersion mirror has only one optical surface. Accordingly, it is difficult to suppress a spherical aberration and a coma aberration, and off-axis aberrations, particularly, the coma aberration are large.